One important consideration in the design of electrical circuits is electromagnetic compatibility (EMC). In particular, electromagnetic fields resulting from noise signals within electrical circuits must be held to within acceptable limits, in order to prevent interference with neighboring circuits. As the density with which electrical circuitry is packaged, and the sensitivity of such circuitry increases, the standards for EMC compliance have become increasingly stringent.
A particular problem within the field of electromagnetic compatibility is to provide effective filtering for a large number of closely spaced conductors, such as conductors associated with high density connectors. A high density connector that is interconnected to a printed circuit board (PCB) must have its pins electrically connected to the conductors of the PCB. Usually, one PCB conductor is mapped to each connector pin. Since PCB conductors propagate high frequency noise currents, it is highly desirable to provide effective filtering against these noise currents. Typically, such filtering is provided by using appropriately valued surface-mountable capacitors to shunt undesirable noise currents to ground, so that the noise currents can return to their sources. Such capacitors have been either embedded onto the high density connector, or they have been soldered onto the PCB, one for each conductor. In either approach, the capacitors are coplanar with the PCB conductors.
Because the capacitors used to shunt noise currents have been coplanar with the PCB conductors, some of the shunted noise currents can be recoupled back into the conductors, due to the magnetic fields from these noise currents being closely located to the conductors. This effect is caused by the phenomena of magnetic field coupling. In addition, where discrete capacitors are placed alongside PCB conductors, a significant amount of PCB real estate (or area) is required. In addition, the conductive traces necessary to connect the capacitors between the circuit and ground can be difficult to route.
Another problem encountered with such approaches is that the shunted high frequency noise currents propagate on one of the PCB surfaces for a short distance before reaching a ground plane, in which the noise currents can return to their sources. This can result in the free space propagation of the electromagnetic fields associated with the noise currents. Such free space propagation can contribute to EMC noncompliance.